Now entertain conjecture of a time
When creeping murmur and the poring dark
Fills the wide vessel of the universe.
[Shakespeare, HENRY V, Act 4, prologue.]
O dark dark dark. They all go into the
dark,
The vacant interstellar spaces, the vacant into the vacant...
[ T. S. Elliott, EAST COKER, first lines of Part III.]
Left to right, Michael
Turner, Saul Perlmutter, Adam Riess, and Brian Schmidt. The last
3 won the Nobel Prize in 2011, for observational confirmation of
Dark Energy, with Turner being the most prominent
theorist. Turner was born in 1949, the other three in
1959, 1969 and 1967, respectively.
The term “dark energy,” echoing Fritz
Zwicky's “dark matter” dating from the 1930s, was coined by
Michael Turner in 1998. If we try to
picture dark energy in quantum field terms, then it is thought
to be very homogeneous, not very dense,
and is not known to interact through any of the fundamental
forces other than gravity.
Since it is quite rarefied, and thus has an effective mass
density that is extremely low — roughly 10−27 kg/m3
— such a field is unlikely ever to be detectable in laboratory
experiments. The reason dark energy can have such a profound
effect on the universe, making up 68% of the universe's energy
density, in spite of being of very low density itself, is that
it uniformly fills otherwise empty space, and thus increases
dramatically in total energy as space-time expands in
volume. The simplest and usual explanation for dark energy
is that it is just Einstein's cosmological constant, Λ, a
fundamental physical constant having
units of energy per unit volume. Independently
of its actual nature, dark energy would need to have a strong
negative pressure to explain the observed acceleration
of the expansion of the universe.
According to Einstein's theory of gravity, the pressure within a
substance contributes to its gravitational attraction for other
things, just as its mass density does. Using the Friedmann–Lemaître–Robertson–Walker
metric, it can be seen that a strong constant negative
pressure in the universe causes an acceleration in universal
expansion if the universe is already expanding, or a
deceleration in universal contraction if the universe is already
contracting. [Actually, you don't need to introduce pressure
to understand the effect of Dark Energy, at all. Just choose the
one of the Friedmann equations which doesn't involve it.]
In standard cosmology, there are three main components of the
universe: matter, radiation and dark energy. Matter is anything
whose energy density scales with the inverse cube of the scale
factor, i.e. ρ ∝ a−3, while radiation is anything
which scales to the inverse fourth power of the scale factor, ρ
∝ a−4. This can be understood intuitively: for an
ordinary particle in a cubic box, doubling the length of a side
of the box decreases the density (and hence energy density) by a
factor of eight (23). For radiation, the decrease in
energy density is greater, because an increase in spatial
distance also causes a red shift.The final component, dark energy, is presumably an
intrinsic property of space, and so has a constant energy
density regardless of the volume under consideration (ρ ∝ a0).
[Text mainly from Wikipedia]
One thing about the expansion of the
universe that is frequently not understood is that it has little
or no effect on bound systems. Atoms are not getting bigger as
space expands, and the effect even on a solar system or a galaxy
is negligible. The reason is that the size of a bound system is
determined by the balance between kinetic and potential energy,
and the balance point depends on fundamental constants of nature
which don't change as space expands. It's a bit different if the
binding is gravitational. Dark energy has an effect which in
weakness today is far, far less than gravity, and it's easy to
do a back-of-the-envelope study of the scale of that effect. To
see how weak the effect of dark energy is, it is useful to
compute the effect of dark energy on the solar orbit of earth
and Pluto. The acceleration on a planet orbiting the sun is -(GM/R2)+kR,
where k = 3.7 × 10-36/s2. Note
that with growing distance between two objects, the effect of
gravity decreases, while the effect of dark energy increases;
for example, doubling the separation between two objects
increases the DE-to-gravity acceleration ratio by a factor of
eight. Now let's plug in the numbers. For earth, the
acceleration from dark energy is 9.3 × 10-23 times
that from the sun's gravity. The corresponding change in the
earth's orbit, compared to an orbit without dark energy (and
with the same orbital velocity), is a tiny 140 nanometers, or
about the size of a virus! For Pluto, the distance from the sun
is 40 times greater than the earth-sun distance; the
corresponding change in Pluto's orbit due to dark energy is 1
micron!
Dark energy can and does have a noticeable,
negative impact on the continual formation of structure in the
universe. A quick survey of the literature indicates that the
first noticeable impact is on the formation of large galactic
clusters. But in the very, very distant future, it
will eliminate
formation of all new structure on a larger scale than dead
stars.
